The present invention relates to an air conditioning device constructed by a waste heat utilization circuit for utilizing waste heat of a heat source in heating of a room to be conditioned in a heat exchanger for heating; a refrigerant circuit in which carbon dioxide is used as a refrigerant and which has a supercritical pressure on a high pressure side; and a cascade heat exchanger which performs heat exchange between a fluid flowing from the heat source to the heat exchanger for heating in the waste heat utilization circuit and a refrigerant of the refrigerant circuit.
Heretofore, this type of air conditioning device is constituted of a waste heat utilization circuit for utilizing waste heat of a heat source of an HEV, an FCV car air conditioner, an FC co-generation system or the like in heating a room to be conditioned in a heat exchanger for heating; a refrigerant including a compressor, a heat absorber disposed externally from the room to be conditioned, a pressure reduction unit and a heat exchanger for cooling which cools the room to be conditioned; a cascade heat exchanger which performs heat exchange between a fluid such as ethylene glycol flowing from the heat source to the heat exchanger for heating in the waste heat utilization circuit and a refrigerant of the refrigerant circuit and the like. The waste heat utilization circuit is provided with a circulation pump. When the circulation pump is operated, the fluid is circulated through the waste heat utilization circuit. The circulation of the fluid through the heat source is controlled by an electromagnetic valve and the like so as to pass the fluid through the heat source in a case where the waste heat of the heat source is utilized in heating the room to be conditioned during a heating operation.
That is, during the heating operation, the fluid heated by heat exchange between the fluid and the heat source is passed through the heat exchanger for heating to heat the room to be conditioned. To heat the heat source during starting, the circulation of the fluid through the heat source is stopped. In the cascade heat exchanger, the heat exchange between the refrigerant compressed by the compressor and the fluid flowing through the waste heat utilization circuit is performed to heat the fluid, and the heated fluid is passed through the heat exchange for heating to heat the room to be conditioned.
Moreover, during the heating operation, in the cascade heat exchanger, the heat exchange between the refrigerant compressed by the compressor and the fluid flowing through the waste heat utilization circuit is performed to heat the fluid. The heated fluid is passed through the heat exchanger for heating to heat the room to be conditioned. Moreover, after reducing pressure of the refrigerant which has radiated heat in the cascade heat exchanger by the pressure reduction unit, the refrigerant is evaporated by the heat absorber disposed externally from the room to be conditioned. To cool the heat source, heating is performed in the cascade heat exchanger in a first stage. In addition, heat from the heat source is applied to the fluid, and utilized in heating the room to be conditioned.
On the other hand, during a cooling operation, without operating the circulation pump of the waste heat utilization circuit, the compressor of the refrigerant circuit is operated, and the pressure of the refrigerant which has radiated heat in the heat exchanger disposed externally is reduced by the pressure reduction unit. Subsequently, the refrigerant is circulated through the heat exchanger for cooling to cool the room to be conditioned.
In addition, in recent years, a problem of global environment has been noticed, and carbon dioxide which is a natural refrigerant has been used as the refrigerant even in this type of air conditioning device. Since carbon dioxide has a characteristic as satisfactory as a global warming coefficient of 1, carbon dioxide has been noted as a substitute for a chlorofluorocarbon-based refrigerant. However, a critical point of the carbon dioxide refrigerant is about 7.31 MPa at 31.1° C., and the pressure of the refrigerant circuit on a high pressure side easily reaches a supercritical region. Especially, when the temperature of the fluid rises by cooling circulation of the heat source, capability of the heat exchange between the refrigerant and the fluid deteriorates. Therefore, after the radiation of the refrigerant, the temperature (at an outlet of the cascade heat exchanger) rises. This causes a problem that cooling/heating capability and efficiency remarkably deteriorate.
To avoid such deterioration of the efficiency, an air conditioning device is developed in which a heat exchanger is additionally disposed in a refrigerant circuit. A refrigerant which has radiated heat in a cascade heat exchanger is passed through the heat exchanger to perform heat exchange between the refrigerant and surrounding air (e.g., air of a room to be conditioned, etc.). In consequence, the refrigerant further radiates heat to thereby increase an enthalpy difference (see, e.g., Japanese Patent Application Laid-Open No. 2002-98430).
However, when the temperature of the air (e.g., the room to be conditioned or the like) to be subjected to the heat exchange between the air and the refrigerant in the heat exchanger rises, such a radiation effect cannot be obtained, and the efficiency disadvantageously remarkably deteriorates. Since the heat exchanger is disposed, the device enlarges and cost increases.
On the other hand, during the heating operation at a low outside air temperature, regardless of the efficiency, the room to be conditioned needs to be heated at an early stage to secure coziness.